Many wireless communication systems may benefit from, or even require, knowledge of geographical location of particular wireless devices. For example, knowledge of user terminal location may be required in some wireless systems such as cellphone networks, cognitive radio systems, etc. One known approach is to use Global Positioning System (GPS) signals for this purpose. Drawbacks of this approach include the need to incorporate a GPS receiver in wireless devices, potential problems with the reception of GPS signals indoors and in the vicinity of tall buildings or in mountainous regions where signal blockage and multipath interference would Occur.
Another approach is to utilize wireless signals to carry out terrestrial ranging, that is calculate the distances between the device transmitting a specific RF signal and the device receiving this RF signal. A particular application is when this RF signal is generated by wireless communication devices and integrated as part of their transmission to determine distances between the devices. Once distances between a target device and two or more reference devices of known locations in the network are determined, the geographical location of the target device can be computed using known triangulation techniques.
The process of determining the distance from a reference wireless device to a target wireless device, is conventionally referred to as range measurements, or ranging. By sending wireless signals from a reference wireless device to a target wireless device and/or vice versa and analyzing timing information comprised in the received signals, time-of-flight information between the two devices can sometimes be estimated. This estimated time of flight may then be used to compute the distance between the reference and target wireless devices. See for example U.S. Pat. No. 5,510,801 to Engelbrecht et al, U.S. Pat. No. 7,126,536 to Rabinowitz et al., U.S. Pat. No. 7,710,321 to Heidari-Bateni et al., and U.S. Patent Application 2009/0010361 to Yang, which are incorporated herein by reference.
One difficulty in using the time-of-flight approach in wireless digital communication systems is the limited accuracy of a receiver timing offset, also referred to in the art as the receiver clock offset, relative to the transmitted signal clock. In a typical receiver, the received signal is sampled, at RF, IF or baseband, by a local clock signal (Rx-clock) typically generated within the receiver. At the transmitter, the signal is created using its own local clock signal (Tx-clock), typically generated within the transmitter. Although Tx-clock and Rx-clock may be suitably well synchronized in frequency, they are typically not synchronized in time with sufficient accuracy. The inaccuracy in time synchronization is hereafter referred to as “Rx timing offset”. One known solution is to use an external reference clock, for example from a GPS, see e.g. U.S. Pat. No. 5,510,801. However, an external reference clock may not always be available or practical. Although methods of estimating the receiver timing offset in the absence of a reference clock are known in the art, such as by correlating a section of the received signal having a known modulation pattern with a copy of the modulation pattern saved at the receiver, these methods often have limited accuracy. In particular, most known methods of estimating the receiver timing offset are limited in accuracy by a sampling time period Ts used in digitizing the received signal, so that the timing offset estimation accuracy is on the order of ±Ts/2, which may not be enough to provide a required ranging accuracy. For example, for a 6 MHz bandwidth signal that may be represented at baseband by a −3 MHz to +3 MHz signal and sampled in the complex plane, the Nyquist sampling frequency is 6 MHz which results in a sampling period of 166 ns, which, in free space and for an electromagnetic signal, corresponds to a propagation distance of 50 m. Once triangulation calculations have taken place, the accuracy achievable will be in the +/−100 m range. To circumvent this limitation, U.S. Pat. No. 7,710,321 discloses a method for estimating the range between two devices wherein the receiver performs multiple samplings of a same received signal with different offsets in the sampling clock phase. This approach effectively increases an effective sampling frequency, providing the receiver with additional information enabling to increase the timing offset accuracy beyond the sampling period. One disadvantage of this method, however, is the large number of additional measurements that the receiver has to perform in this method, with the proportionally increased computation requirements, so that the method does not scale well if the required increase in the timing offset accuracy is, for example, 10-fold or 100-fold.
Accordingly, an object of the present invention is to provide a method and apparatus for determining a receiver timing offset with an accuracy better than the receiver sampling period, without requiring considerable changes to the receiver sampling process, and which could be used in ranging, geo-locationing and radar-type applications for accurate determination of distances and locations using multicarrier signals.